Thermosensitive resistance element



B. B. JACOBSEN EI'AL 2,651,699

THERMOSENSITIVE RESISTANCE ELEMENT Sept. 8, 1953 illlliliillla J Filed Jan. 25, 1952 l/V VEN TO R5 5. 5. JACOBSE/V C- T. DALY a 1.. THOMAS H. WOL F5 0M 5 C. SHEPARD a GREEN A ttnrne y Patented Sept. 8,

THERMOSENSITIVE RESISTANCE ELEMENT Bent Bulow J acobsen, Charles Trevor Daly, David Lane Thomas, Henry Wolfson, Stanley Garden Shepard, and David Green, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

Application January 25, 1952, Serial No. 268,224 In Great Britain January 30, 1951 1 Claim.

The present invention relates to electric thermo-sensitive resistance elements of the kind known as thermistors.

One important application of thermistors is in automatic gain control arrangements employed in communication systems, in which the thermistor forms part of the gain control network associated with an amplifier, and the heating of the thermistor is controlled by a pilot current transmitted with the communication signals.

For this purpose it is desirable that the thermistor should have a high thermal capacity, and low heat losses. The object of the present invention, therefore, is to improve the design of thermistors so that they meet these requirements more closely.

This object is achieved according to the invention by providing an indirectly heated thermistor comprising in an evacuated envelope, a thermistor body consisting of a bead of thermosensitive resistance material having a pair of terminal leads, and enclosed in a heating coil of such length as also to enclose a section of each of the said leads and terminal wires for the said heating coil, each of which wires is coiled in such manner as to provide a choke for reducing the rate of conduction of heat along the said wire.

The invention will be explained with reference to the figure of the accompanying drawing, which shows an indirectly heated thermistor according to the invention, together with the lead-out conductors, the thermistor being sectioned in order to show its parts clearly.

The thermistor comprises a small bead l of suitable thermosensitive resistive material having embedded therein two fine Wires 2 and 3 serving as the terminal leads for the bead. These wires are soldered or otherwise conductively attached respectively to a pair of lead-out conductors 4 and 5 of heavier gauge. The thermistor bead l is embedded at the centre of a cylinder 6 of electrically non-conducting cement carrying on its outside surface a helical heater winding I wound closely to the bead l. The assembly is slipped inside a thin cylindrical metal tube 8, preferably of silver. The assembly consisting of the elements l, 6, l and 8 will be called, for convenience, the thermistor body.

The terminal leads 9 and In for the heater winding '1 are wound into coils for the purpose of acting as heat conduction chokes. These terminal leads are soldered, or otherwise conductively attached respectively to two lead-out conductors II and I2 of heavier gauge which also act as supports for the thermistor. The whole assembly is mounted in the usual way in an evacuated envelope (not shown).

The principal feature of the invention is the use of the coiled terminal leads 9, It for the heater winding by which the rate of conduction of heat along these leads is reduced, so that there will be a small temperature gradient between the thermistor body and its supports. Another feature is bringing out both the bead conductor wires, 2, 3 through the same end of the heater winding, and increasing the length of this winding so that it overlaps a considerable length of these wires, as shown in the figure, whereby there will be no temperature gradients in the bead, and the loss oi heat by conduction along these Wires will also be reduced.

The metal tube 8 should have a polished outer surface to reduce heat radiation, the preferred metal bein silver, as already stated. This tube increases the thermal capacity of the thermistor, and the dimensions should be chosen so that by its presence a greater increase in thermal capacity than in the heat losses is produced. The use of a metal enclosure for the thermistor is known, and is described in British patent specification No. 555,563.

The terminal leads 2 and 3 for the thermistor bead should be made from the smallest gauge wire which is practicable, and the material of this wire should preferably be a metal or alloy which has the highest possible ratio or" the thermal to the electrical resistance. This feature is also disclosed in the above-mentioned patent specification.

A further desirable point is that the diameter of the non-conducting cylinder 6 should be as small as possible in order to have the minimum possible clearance between the heating winding i and the bead l.

Finally, the envelope (not shown) in which the thermistor is enclosed should be evacuated as highly as possible.

While the principles of the invention have been described above in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made only by way of example, and not as a limitation on the scope of the invention.

What we claim is:

An indirectly heated thermistor comprising, in an evacuated envelope, a thermistor body consisting of a bead of thermosensitive resistance material, a pair of terminal leads extending parallel to each other in the same direction from one end of the bead, a heating coil enclosing said thermistor body and of such length as to enclose a section of said leads, and terminal wires for the said heating coil, each of which Wires is coiled in such manner as to provide a choke for reducwire'.

BENT BULOW JACOBSEN. .CHARLES TREVOR DALY. DAVID LANE THOMAS. I-IENRY WOLFSON. STANLEY GARDEN SHEPARD. DAVID GREENE, A

Number 'ing the rate of conduction of heat along the said 5 2,396,196

Number UNITED STATES PATENTS Name Date Pearson Mar. 5, 1946 FOREIGN PATENTS Country Date Great Britain Aug. 28, 1943 France Nov. 3, 1947 

